Critical review of design principles for rock support in burst-prone ground – time to rethink!

In the 1980s, severe rockbursting in underground mines, in Canada, Chile, South Africa, and other countries, caused many fatalities and was recognised as an unacceptable work place hazard. Since then huge advances have been made in seismic monitoring, energy release control by improved mining methods, ground control with innovative and effective rock support systems, and advanced risk hazard management procedures. As a result, many burst-prone mines have achieved outstanding safety records and the industry has benefited by developing means of accessing highly stressed orebodies at depth. In 1995, the Canadian Rockburst Research Program was completed and guidelines for rock support selection were published. Since then many researchers around the world have further advanced the state-of-the-art of support selection and many entrepreneurs have provided industry with a sophisticated tool box full of effective rock retention and support components and systems. Most of these advances and the design of these tools, however, are based on some fundamental assumptions, i.e. that seismic events create ground motions that damage excavations, and thus load and potentially damage the ground support. It is therefore often implied that the source of energy which causes damage stems from mining-induced seismic events. In this paper, we critically assess the support design guiding principle, which leads to the identification of three facts with serious practical implications: (1) standard, assumed ground motion patterns are unacceptably flawed; (2) ground motion-related energy transfer mechanisms rarely constitute the main source of support loading; and (3) dynamically stressed rock fails by brittle, extensional failure causing disproportional excavation convergences and support damaging deformations. The keynote papers presented at this symposium in Australia and at the Eighth International Symposium on Rockbursts and Seismicity in Mines in Russia describe these flaws and recent advances to overcome related deficiencies. It is concluded that the primary damage mechanism often is a seismically triggered strainburst or a dynamically loaded strainburst, and damage is rarely dominated by momentum transfer or dynamic shaking. Consequently, rockburst damage is primarily related to the state of stress and the local mine stiffness at the potential damage location and not or only indirectly related to the seismic ground motion. Field evidence is used to illustrate and support this interpretation.

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